According to the prior art (U.S. Pat. No. 3,105,744), calcination of the purified hydrolyzate obtained by the sulfate process is carried out primarily in rotary kilns where product temperatures of from 800.degree. to 1100.degree. C. are reached. Heating of the rotary kilns is by fuel oil or fuel gas and involves high energy consumption.
The hydrolyzate normally contains 7-9% by weight H.sub.2 SO.sub.4, based on TiO.sub.2, as an unwanted secondary constituent. SO.sub.2 and SO.sub.3 are formed therefrom during the calcination step, H.sub.2 SO.sub.4 being formed by reaction of the SO.sub.3 with steam. In addition to steam, nitrogen, oxygen, carbon dioxide and dust, therefore, the offgases issuing from the kilns used for calcination also contain SO.sub.2 and SO.sub.3 or H.sub.2 SO.sub.4.
The properties of the TiO.sub.2 pigments, particularly the relative content of the two modifications, anatase and rutile, and also the particle size of the pigment crystals depend to a large extent upon the calcination conditions.
The addition of excess air (secondary air) to the smoke gases formed during combustion of the fuels promotes the development of a certain temperature profile in the rotary kiln which is optimal for the particular product. To monitor this optimal temperature profile and, hence, the calcination conditions, the product temperatures of the TiO.sub.2 pigment formed and/or the gas temperatures in the kiln are normally measured and compared with predetermined ideal values for the particular pigment quality required. In the event of deviations from the preset values, the quantity of fuel and, optionally, the quantity of air is increased or reduced according to requirements.
However, difficulties are involved in accurately measuring the temperatures in the rotary kilns and in keeping them constant.
Accordingly, the calcination conditions always have to be readjusted whenever deviations from the desired quality are detected during the examination of pigment samples.
On account of the relatively long time interval between sampling and readjustment of the calcination conditions, variations in the quality of the TiO.sub.2 pigment discharged from the kiln are unavoidable.
Even in cases where the offgases issuing from the rotary kilns used for calcination are partly recycled to the combustion chambers (U.S. Pat. No. 3,105,744), variations in the quality of the TiO.sub.2 pigment are unavoidable on account of the inertia of the described method of controlling the calcination conditions.
In addition, the SO.sub.2 content of the offgases is subject to considerable variation in this process, giving rise to problems in the now necessary removal of the SO.sub.2 content of the calcination offgases which are released into the atmosphere.